1. Field of the Invention
The present invention relates to a radio transceiver, particularly for use in a time division duplex system. In specific embodiments, the invention relates in particular to a double superheterodyne transponder terminal for receiving and transmitting AIS (Automatic Identification System) signals in the maritime VHF band (156.025 to 162.025 MHz).
2. Description of the Related Art
The rapid uptake of maritime AIS equipment is being driven by the navigational safety benefits and by governmental homeland security mandates in many regions. Whilst large international shipping over 300 tonnes has been obliged to carry class A category AIS for several years, smaller craft have only carried the class B category AIS on a voluntary basis until recent mandates came into force. Class B AIS equipment has been mostly targeted at the leisure boating market and hence has been designed for low cost, high volume commercial manufacture. This, coupled with the desire of many OEMs to build AIS modules into other equipment such as chart plotter displays, has driven the trend towards ever smaller AIS module designs.
FIG. 1 shows a conventional low cost commercial class B AIS transponder. Generally, the transponder 10 shown in FIG. 1 comprises two independent superheterodyne receiver chains 12, 14 and one directly modulated transmitter chain 16.
More specifically, the transponder 10 comprises a VHF antenna port 18, connected to a transmit/receive radio frequency (RF) switch 20. On the receive side, the switch 20 is connected through a first surface acoustic wave (SAW) filter 22 and a low noise amplifier 24 to a passive splitter 26. The splitter 26 is connected to the receiver chains 12, 14.
Thus, a signal entering the first receiver chain 12 is passed to a second SAW filter 28, and then to a first RF mixer 30, where it is downconverted to a first intermediate frequency (IF). The resulting IF signal is passed through a first IF amplifier 32, an IF crystal filter 34, and a second IF amplifier 36 to an integrated second IF mixer and demodulator 38, where it is further downconverted. The resulting baseband signal is output at an output terminal 40.
Similarly, a signal entering the second receiver chain 14 is passed to a third SAW filter 42, and then to a second RF mixer 44, where it is downconverted to a second intermediate frequency (IF). The resulting IF signal is passed through a third IF amplifier 46, an IF crystal filter 48, and a fourth IF amplifier 50 to a second integrated second IF mixer and demodulator 52, where it is further downconverted. The resulting baseband signal is output at a second output terminal 54.
The source of the required local oscillator signals is a quartz reference clock 56, which is connected to a dual PLL synthesiser 58. The dual PLL synthesiser 58 is connected to a first Voltage Controlled Oscillator (VCO) 60, for generating a local oscillator signal at a first local oscillator frequency, and to a second Voltage Controlled Oscillator (VCO) 62, for generating a local oscillator signal at a second local oscillator frequency. The local oscillator signal at the first local oscillator frequency is supplied to a second input of the first RF mixer 30, and the local oscillator signal at the second local oscillator frequency is supplied to a second input of the second RF mixer 44.
Thus, by appropriate selection of the signals at the first and second local oscillator frequencies, signals on the required AIS channels can be detected.
In the transmit chain 16, a clock signal at a known frequency is supplied on one input 70 of a dual fractional-N PLL synthesiser 72, which receives a modulation word from a microprocessor on its second input 74. The dual fractional-N PLL synthesiser 72 can be controlled so that it generates a signal on a chosen one of the two AIS channels in use. The resulting output signal is passed through a transmit path Voltage Controlled Oscillator (VCO) 76, and then through a first RF power amplifier 78 and second RF power amplifier 80 to a harmonic low pass filter 82, before passing through the switch 20 to the antenna 18.
The two receivers 12, 14 normally monitor the two default AIS channels AIS1 and AIS2 collecting position report broadcasts from other AIS terminals within VHF radio range. On a periodic basis the class B AIS transponder 10 transmits its own geographical position data, derived from an on-board GNSS receiver. The timing of this automatic transmission is determined by an internationally agreed protocol based on the Carrier-Sense TDMA system.